Does survival lessen after 21 days?

[jenren]

My son is incubating 9 chicken eggs, 1 hatched yesterday and is doing great. But so far that is the only one we have. I noticed 3 of the eggs rocking last night, but nothing yet. How many days after the 21st day can they hatch? Thank you so much!!! Jennifer

 

[hatchcrazzzy]

i usually leave them for 3 or 4 days